Transparent amorphous metal oxide coatings may be applied to substrates or over-coated on top of other previously deposited layers. Such, layers may exhibit high transparency with electrical conductivity ranging from semiconducting to highly conducting, Accordingly, these coatings, or “thin films” as they are often referred, to, may be used for a wide range of opto-electronic applications. For example, thin, films may be used in the manufacture of electronic devices, such as, liquid crystal displays (LCDs), and touch panel devices (e.g., personal digital assistants (PDAs) and electronic controllers), photovoltaic solar cells and transparent thin film transistors to name only a few examples.
Known deposition techniques may be implemented during the manufacture process to deposit an amorphous metal oxide as the thin film on a substrate. Indium Zinc Oxide (also commonly referred to as IZO) is often used because it can be deposited at low temperatures, results in smooth films, can be readily etched, and exhibits thermal stability. However, other amorphous metal oxides (e.g., Zn—Sn—O, In—Ga—O, and Zn—In—Sn—O) may also be used depending on the desired properties of the thin film, cost, operational parameters, and other factors.
In order to achieve the desired opto-electronic properties of the thin film, the relative quantities of metal alloy and oxygen introduced during the deposition process are typically determined on a “trial and error” basis for a particular deposition chamber and target material stoichiometry. Once determined, it was believed that the amount of oxygen could not be varied without affecting the opto-electronic properties of the thin film. The manufacture process of these thin films has been constrained by fixed processing parameters.
The reported optimum Zn/(Zn+In) content in the target/coating necessary to get the optimum properties varies dramatically within the reported literature. In Naghavi, et al, (Electrochimica Acta 46 (2001) 2007-2013) the authors report with Pulsed Laser Deposition (PLD) that a 50% Zn content gives the best properties. Their test was at a fixed oxygen content in the PLD atmosphere and a substrate temperature of 500° C. In Minami et al, (Thin Solid Films 290-291 (1996) 1-5) the authors show that the optimum Zn content in the coatings varies with the substrate temperature. At room temperature substrate the preferred Zn content is 24.5% while at a substrate temperature of 350° C. the preferred composition is 42.2% Zn, The Minami tests were conducted at a fixed oxygen composition in the sputter environment. The researchers examined pressure effects only at the preferred compositions at each temperature, in Minami, et al (Jpn. J. Appl. Phys. Vol. 24 (1995) Pt. 2, No. 8A) the composition of Zn in IZO is evaluated in a chamber absent of oxygen and at a fixed pressure. The composition is varied to determine the optimum conductivity then other process conditions are varied such as oxygen flow, total pressure and substrate temperature. They found that additional oxygen was detrimental at their optimum composition of 30% Zn. The Minami research also resulted in coatings with extremely high refractive indices which can be detrimental in some optical applications. In Naghavi et al, (Thin Solid Films 360 (2000) 233-240) the researchers add hydrogen to the list of process variables in a PLD process and report that the optimum Zn content is at 60%. The effect of any given process variable is not readily apparent from this study because changes in one process variable often affects multiple aspects of the coating composition and there are therefore insufficient independent variables to properly determine the trends with process conditions. In Moriga, et al, (Thin Solid Films 486 (2005) 53-57) the researchers use PLD to study IZO. They examined the properties of the coatings at two oxygen levels and one argon/oxygen mixture. The slope of resistivity with composition varies with which gas is in the chamber but the chamber gas also affects the composition of the coating. The researchers do not report the effects in terms of the coating composition but rather in terms of the target composition. Since the coating does not match the target equivalently with the different chamber gasses it is impossible to uniquely identify the effects of the different process variables. The optimum resistance values occur at 40% Zn content in the target irrespective of the sputtering gas. Finally, in Taylor, et al (Meas. Set, Technol. 16 (2005) 90-94) the researchers show the optimum composition at 30% Zn content. Their study did not look at the effects of the process just the composition of the coating.
All of these studies point to different Zn contents and the wide range of process conditions do not lead one to determine how to optimize the IZO coating at a given composition. There is almost a “trial-and-error” or “empirical nature” to these approaches which has limited the useful compositions of IZO.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.